1. Field of the Invention
The present invention relates to a system and method for optimal sensing of cardiac events. In particular, the present invention relates to a system and method for controlling a refractory period for a cardiac monitoring or therapy device to achieve optimal sensing of cardiac events.
2. Related Art
The system and method of the present invention are intended to be incorporated into a conventional pacemaker or defibrillator. Both devices are described below. A modern implantable defibrillator includes a pulse generator, which is connected to a patient's heart, via an electrode lead system having one or more electrode leads respectively carrying one or more electrodes. The pulse generator supplies stimulation pulses to the heart and is controlled by a control unit. A detector is arranged to sense heart activity via the electrode lead system, and to send detected information to the control unit for controlling the pulse generator in accordance therewith. An electrode switching unit is devised to connect different combinations of electrodes of the leads of the electrode lead system to the pulse generator according to a defined pattern or in some optional way. A conventional defibrillator senses when a heart is undergoing fibrillation and delivers a defibrillation shock to the heart. Defibrillators can also deliver tachyarrhythmia pacing therapy to the heart, known as antitachycardia pacing. This pacing consists of delivery of a train of pace pulses to the heart.
The operation of such a conventional defibrillator is described in detail in U.S. Pat. No. 5,007,422 to Pless et al. And in U.S. Pat. No. 5,632,267 to Hognelid et al. which are both incorporated herein, in their entirety, by reference.
Similarly, modern pacemakers include a pulse generator, which is connected to a patient's heart via an lead system having one or more leads with one or more electrodes. Typically, the leads are bipolar where localized sensing is required. The pulse generator provides pacing pulses to the heart and is controlled by a control unit. A detector is used to sense heart activity to control the pulse generator. Pacemakers traditionally delivery bradycardia pacing pulses to the heart. Antitachycardia pacing pulses are typically delivered at a much higher rate than bradycardia pacing pulses.
Modem pacemakers may include complex stimulation pulse generators as well as cardiac event sensors that can pace or sense in the atrium, the ventricle, or both the atrium and ventricle of the heart. Further, such pacemakers include telemetry capabilities so that the activity of the heart and pacemaker can be transmitted to an attending physician or cardiologist. Advantageously, such pacemakers are also programmable so that the same telemetry capabilities can be used by the attending physician or cardiologist in order to adjust the pulse characteristics such as width and voltage, and parameters associated with operation of the pacemaker. Such parameters not only influence the rate at which the pacemaker's stimulation pulses are generated, but also control the pacemaker's basic mode of operation, i.e., the heart chamber that is paced, as well as the heart chamber that is sensed. Hence, modern pacemakers offer great versatility in the manner of their use.
Referring to FIG. 8, there is shown a simplified representation of one way that an implanted pacemaker 802 may make electrical contact with the heart. It is well known in the art that there are other ways to connect pacemakers to the heart. Further, the method for implanting leads for an implantable defibrillator is also well known in the art.
FIG. 8 shows the four (4) chambers of the heart, namely, the right atrium 822, the right ventricle 824, the left atrium 826, and the left ventricle 828. The atrium chambers function primarily as reservoirs into which incoming blood is received. The ventricles function primarily as pumping chambers to pump the blood away from the heart to a specific destination.
Right atrium 822 has an S-A node (not shown) that begins the electrical impulse that spreads in wave fashion to stimulate both right atrium 822 and left atrium 826. It is this electrical impulse that causes depolarization of the muscle tissue that forms the walls of the atria, thereby causing atrial contraction to occur. Right atrium 822 also includes an A-V node (not shown) which is stimulated by the electrical impulse propagated from the S-A node. Upon stimulation, and after a short pause (typically about 0.1 seconds), the A-V node initiates an electrical impulse that starts traveling down an A-V bundle (not shown). This A-V bundle branches and distributes the electrical impulse throughout the myocardium or heart muscle, thereby causing the ventricles to depolarize and contract.
FIG. 8 depicts the use of two (2) bipolar leads 804 and 806, each being directed into a separate chamber of the heart. A bipolar lead comprises a single filar strand that includes two (2) electrically insulated conductors. For example, lead 806 includes a first conductor 808 that is electrically connected to a distal tip 810 of the lead. Distal tip 810 is typically placed in a cavity of right atrium 822 referred to as an atrial appendage 812. At a known distance from distal tip 810, an electrode ring 814 is electrically connected to another conductor 816 of bipolar lead 806. Similarly, a distal tip 818 and a conductive ring 820 are associated with bipolar lead 804 which is placed in the apex of right ventricle 824. The manner in which leads 804 and 806 are inserted into the heart, as well as the manner in which pacemaker 802 is implanted in the body of a patient, are well known in the art.
With every natural or intrinsic heart activity, an electrogram can be detected by the pulse generator via the lead. In the waveform of the electrogram, one can identify the intrinsic deflection, a rapid biphasic voltage change, corresponding to the depolarization wave front passing under the electrode(s). In the waveform of the electrogram, there are several other components which are sometimes seen in addition to the intrinsic deflection, including repolarization potentials, QRS in the atrial electrogram, myopotentials in the unipolar pacing system (generated by skeletal-muscle contraction), and electromagnetic interference. The intrinsic deflection is the most important component of natural heart activity detected by the electrode(s).
FIG. 9 shows a representation of the various waveforms that are generated, as sensed by skin electrodes placed on the chest. A P-wave represents the depolarization of both atria. The QRS-wave, commonly referred as the QRS complex, represents the electrical impulse as it travels from the A-V node to the various fibers branching from the left and right bundle branches as it is distributed into the myocardial cells, thereby causing ventricular depolarization. The T-wave represents the repolarization of the ventricles so that they may be stimulated again. Repolarization of the atrium is usually not sensed because it occurs about the same time as the QRS complex, and any signals representative of atrial repolarization are therefore masked out by the QRS complex.
One cardiac cycle is represented by a P-wave, a QRS complex, and a T-wave. This cardiac cycle is repeated continuously as the heart pumps blood. In summary, the P-wave represents depolarization of the atria. The QRS complex, sometimes referred to as simply an R-wave, represents the depolarization of the ventricles. Depolarization/contraction of the atria, followed a short time thereafter by depolarization/contraction of the ventricles, are the cardiac events that must occur if the heart is to efficiently perform its function as a pump in distributing blood throughout the body.
Returning now to a discussion of pacemakers, there are two types of pacemakers, fixed or asynchronous pacemakers and demand pacemakers. In an asynchronous or fixed rate pacemaker system, the control unit acts like a counter, which counts down, for example, every millisecond (ms). When the counter reaches zero, it commands an output circuit to deliver an electrical stimulus to the heart muscle. At the same time, the stimulus resets the counter to its original value. The time between two electrical impulses is referred to as an escape interval. The escape interval corresponds to the paced heart rate or basic rate. The escape interval is determined by the value to which the counter is set. In the fixed rate system, electrical stimuli arrive at fixed intervals, irrespective of natural or intrinsic heart activity.
An asynchronous pacemaker competes with a patient's natural heart activity and can sometimes stimulate the heart in a vulnerable period which could induce arrhythmias. However, if the counter is reset at the moment of spontaneous cardiac activity, competition between the pacemaker and the natural heart rhythm will be avoided. As such, demand pacemakers having the ability to sense intrinsic heart activity have been developed.
A demand pacemaker uses a sensing amplifier to detect intrinsic heart activity, so that the pacemaker delivers an electrical stimulus, also referred to herein as a pace pulse, to the heart only when the natural heart rate drops below the rate set by the counter. Thus, if the pacemaker does not detect intrinsic heart activity by the time the counter reaches zero, the pacemaker will deliver an electrical stimulus to the heart. The demand pacemaker ensures that the lowest possible heart rate is the basic rate of the pulse generator. This rate is sometimes referred to as the lower rate or backup rate. If the pulse generator is inhibited from delivering an electrical stimulus to the heart, because the sensing amplifier senses intrinsic heart activity, the counter is reset and a new escape interval begins. Similarly, if no intrinsic heart activity has been detected, and the pulse generator delivers an electrical stimulus to the heart, the counter is reset at the same time as delivery of the stimulus.
In a demand pacemaker, wherein a stimulating pulse is provided by the pacemaker only when a natural cardiac event fails to occur within a prescribed escape time interval, the escape time interval can be adjusted as a function of the reference interval measurement, and thereby adjust the pacing rate as a function of physiological need, as shown in U.S. Pat. No. 4,712,555 to Thornander et al.
In a demand pacemaker it is common to define an escape interval during which activity within the heart is sensed. If a natural cardiac event occurs during this escape interval, that is if a natural P-wave or R-wave is sensed, then a corresponding stimulating pulse need not be generated. This mode of operation allows the heart to function in its natural state, if it is able. Further, a demand pacemaker helps conserve the limited power stored within the battery of the pacemaker. One problem with asynchronous pacemakers is that they cannot change the stimulation pattern. The asynchronous pacemaker is set to a fixed rate and is refractory to intrinsic cardiac activity throughout its cycle.
In demand pacemakers and in defibrillators, both having sensing capabilities, signals such as interference, repolarization signals, and other far field signals may appear to the pulse generator to be intrinsic cardiac events, thereby causing the pulse generator to reset the counter. However, there is a certain period after heart activity during which new heart activity cannot physiologically occur. During this period, any cardiac event sensed by the pulse generator cannot correspond to new cardiac activity, and as such, the pacemaker counter should not be reset and a defibrillation shock should not be applied. This period is called the "refractory period" of the pulse generator.
The nominal refractory period for conventional pacemakers starts either with sensing of an intrinsic cardiac event or delivery of a pace pulse, and generally lasts for approximately 300 to 350 ms following delivery of a pace pulse or approximately 130 to 150 ms following sensing of an intrinsic cardiac event. The nominal refractory period for a defibrillator starts with delivery of a defibrillation shock to the heart, and generally lasts approximately 500 to 1000 ms.
In a defibrillator, the pulse generator is trying to detect very fast heart rates, greater than 300 beats per minute. As such, the pulse generator must be alert as much as possible. Using a fixed refractory period limits the fastest heart rate that can be sensed. Thus, there is a need to minimize the refractory period as much as possible.
A pacemaker typically operates at approximately 70-100 beats per minute, much slower than the rate of a heart in fibrillation. In this case, if the refractory period is too long, it generally does not pose a problem. However, if the refractory period is too short, then oversensing occurs, and necessary pacing pulses may not be delivered to the heart. As such, there is a need to monitor the cardiac event so that the refractory period does not end prior to the end of cardiac event. This prevents the same cardiac event from being sensed more than once.
The simple approach of programming refractory periods to a fixed length of time can lead to refractory periods which are longer than necessary, thus wasting valuable sensing opportunities. Alternatively, the approach may lead to refractory periods which are shorter than necessary, leading to multiple sensing of a single cardiac event. What is needed is a method for setting the refractory period such that the refractory period ends when the cardiac activity has ceased.